Mail User Agent

The mail user agent is the program that acts as the interface between the user
and mail transfer agent. The sendmail program is a mail transfer agent.
The Solaris operating system supplies the following mail user agents.

/usr/bin/mail

/usr/bin/mailx

/usr/dt/bin/dtmail

Mail Transfer Agent

The mail transfer agent is responsible for the routing of mail messages and the resolution
of mail addresses. This agent is also known as a mail transport agent.
The transfer agent for the Solaris operating system is sendmail. The transfer agent performs
these functions.

Accepts messages from the mail user agent

Resolves destination addresses

Selects a proper delivery agent to deliver the mail

Receives incoming mail from other mail transfer agents

Local Delivery Agent

A local delivery agent is a program that implements a mail delivery protocol. The following
local delivery agents are provided with the Solaris operating system.

The UUCP local delivery agent, which uses uux to deliver mail

The local delivery agent, which is mail.local in the standard Solaris release

Mailers and sendmail

Mailer is a sendmail-specific term. A mailer is used by sendmail to
identify a specific instance of a customized local delivery agent or a customized
mail transfer agent. You need to specify at least one mailer in your
sendmail.cf file. For task information, refer to Changing the sendmail Configuration in Chapter 13, Mail Services (Tasks). This section provides
a brief description of two types of mailers.

Names in the $=U class are sent to uucp-old. uucp is the obsolete name for this mailer. The uucp-old mailer uses an exclamation-point address in the headers.

uucp-new

Names in the $=Y class are sent to uucp-new. Use this mailer when you know that the receiving UUCP mailer can manage multiple recipients in one transfer. suucp is the obsolete name for this mailer. The uucp-new mailer also uses an exclamation-point address in the headers.

If MAILER(smtp) is also specified in your configuration, two more mailers are defined.

Mail Addresses

The mail address contains the name of the recipient and the system to which
the mail message is delivered. When you administer a small mail system that
does not use a name service, addressing mail is easy. The login names
uniquely identify the users. Complexity is introduced if you are administering a mail
system that has more than one system with mailboxes or that has one
or more domains. Additional complexity can be generated if you have a UUCP
(or other) mail connection to servers outside your network. The information in the following
sections can help you understand the parts and complexities of a mail address.

Domains and Subdomains

Email addressing uses domains. A domain is a directory structure for network address
naming. A domain can have one or more subdomains. The domain and subdomains of
an address can be compared to the hierarchy of a file system.
Just as a subdirectory is considered to be inside the directory above it, each
subdomain in a mail address is considered to be inside the location to
its right.

The following table shows some top-level domains.

Table 14-7 Top-Level Domains

Domain

Description

com

Commercial sites

edu

Educational sites

gov

United States government installations

mil

United States
military installations

net

Networking organizations

org

Other nonprofit organizations

Domains are case insensitive. You can use uppercase, lowercase, or mixed-case letters in
the domain part of an address without making any errors.

Name Service Domain Name and Mail Domain Name

When you are working with name service domain names and mail domain names,
remember the following.

By default, the sendmail program strips the first component from the NIS or NIS+ domain name to form the mail domain name. For example, if an NIS+ domain name were bldg5.example.com, its mail domain name would be example.com.

Although mail domain addresses are case insensitive, the NIS or NIS+ domain name is not. For the best results, use lowercase characters when setting up the mail and NIS or NIS+ domain names.

Typical Format for Mail Addresses

The part of the address to the left of the @ sign is
the local address. The local address can contain the following.

Information about routing with another mail transport (for example, bob::vmsvax@gateway or smallberries%mill.uucp@gateway)

An alias (for example, iggy.ignatz)

Note - The receiving mailer is responsible for determining what the local part of the
address means. For information about mailers, refer to Mailers and sendmail.

The part of the address to the right of the @ sign
shows the domain levels, which is where the local address resides. A dot
separates each subdomain. The domain part of the address can be an organization,
a physical area, or a geographic region. Furthermore, the order of domain information
is hierarchical, so the more local the subdomain, the closer the subdomain is
to the @ sign.

Route–Independent Mail Addresses

Mail addresses can be route independent. Route-independent addressing requires the sender of an
email message to specify the name of the recipient and the final destination.
A high-speed network, such as the Internet, uses route-independent addresses. Route-independent addresses can have
this format.

user@host.domain

Route-independent addresses for UUCP connections can have this address format.

host.domain!user

The increased popularity of the domain-hierarchical naming scheme for computers is making route-independent
addresses more common. Actually, the most common route-independent address omits the host name
and relies on the domain name service to properly identify the final destination of
the email message.

user@domain

Route-independent addresses are first read by searching for the @ sign. The domain
hierarchy is then read from the right (the highest level) to the left
(the most specific part of the address to the right of the @
sign).

Mailbox Files

A mailbox is a file that is the final destination for email messages.
The name of the mailbox can be the user name or the identity
of a specific function, such as the postmaster. Mailboxes are in the /var/mail/username
file, which can exist either on the user's local system or on a
remote mail server. In either instance, the mailbox is on the system to
which the mail is delivered.

Mail should always be delivered to a local file system so that the
user agent can pull mail from the mail spool and store it readily
in the local mailbox. Do not use NFS-mounted file systems as the destination
for a user's mailbox. Specifically, do not direct mail to a mail client
that is mounting the /var/mail file system from a remote server. Mail for
the user, in this instance, should be addressed to the mail server and
not to the client host name. NFS-mounted file systems can cause problems with
mail delivery and handling.

The /etc/mail/aliases file and name services such as NIS and NIS+ provide mechanisms for
creating aliases for electronic mail addresses. So, users do not need to know
the precise local name of a user's mailbox.

The following table shows some common naming conventions for special-purpose mailboxes.

User names can be identified as full names with a dot (or
an underscore) that separates the first and last names. Alternately, user names can be
identified by a first initial with a dot (or an underscore) that separates
the initial and the last name.

postmaster

Users can address questions and report
problems with the mail system to the postmaster mailbox. Each site and domain should
have a postmaster mailbox.

MAILER-DAEMON

sendmail automatically routes any mail that is addressed to the
MAILER-DAEMON to the postmaster.

aliasname-request

Names that end in -request are administrative addresses for
distribution lists. This address should redirect mail to the person who maintains the
distribution list.

owner-aliasname

Names that begin with owner- are administrative addresses for distribution lists. This
address should redirect mail to the person who handles mail errors.

owner-owner

This alias is
used when no owner-aliasname alias exists for errors to be returned to.
This address should redirect mail to the person who handles mail errors. This
address also should be defined on any system that maintains a large number of
aliases.

local%domain

The percent sign (%) marks a local address
that is expanded when the message arrives at its destination. Most mail systems
interpret mailbox names with % characters as full mail addresses. The %
is replaced with an @, and the mail is redirected accordingly. Although many people
use the % convention, this convention is not a formal standard. This convention
is referred to as the “percent hack.” This feature is often used to help
debug mail problems.

Starting with sendmail version 8, the envelope sender for mail that is sent
to a group alias has been changed to the address that is expanded
from the owner alias, if an owner alias exists. This change enables any
mail errors to be sent to the alias owner, rather than being returned
to the sender. With this change, users notice that mail that was sent
to an alias looks as if the mail came from the alias owner,
when delivered. The following alias format helps with some of the problems
that are associated with this change.

In this example, the mygroup alias is the actual mail alias for the
group. The owner-mygroup alias receives error messages. The mygroup-request alias should be used for
administrative requests. This structure means that in mail sent to the mygroup alias,
the envelope sender changes to mygroup-request.

For large sites, the mail alias typically defines the location of a mailbox. Providing
a mail alias is like providing a room number as part of the
address for an individual at a large corporation that occupies multiple rooms. If
you do not provide the room number, the mail is delivered to a
central address. Without a room number, extra effort is required to determine where
within the building the mail is to be delivered. So, the possibility of
an error increases. For example, if two people who are named Kevin Smith
are in the same building, only one of them might get mail. To
correct the problem, each Kevin Smith should have a room number added to
his address.

Use domains and location-independent addresses as much as possible when you create mailing
lists. To enhance portability and flexibility of alias files, make your alias entries
in mailing lists as generic and system independent as possible. For example, if
you have a user who is named ignatz on system mars, in domain example.com,
create the alias ignatz@example instead of ignatz@mars. If user ignatz changes the
name of his system but remains within the example domain, you do
not need to update alias files to reflect the change in system name.

When you create alias entries, type one alias per line. You should have
only one entry that contains the user's system name. For example, you could
create the following entries for user ignatz.

ignatz: iggy.ignatz
iggyi: iggy.ignatz
iggy.ignatz: ignatz@mars

You can create an alias for local names or domains. For example, an
alias entry for user fred, who has a mailbox on the system mars
and is in the domain planets, could have this entry in the
NIS+ aliases table.

fred: fred@planets

When you create mail lists that include users outside your domain, create the
alias with the user name and the domain name. For example, if you
have a user who is named smallberries on system privet, in domain example.com, create
the alias as smallberries@example.com. The email address of the sender is now automatically
translated to a fully qualified domain name when mail goes outside the user's
domain.

The following list describes methods for creating and administering mail alias files.

You can create mail aliases for global use in the NIS+ mail_aliases table, the NIS aliases map, or in local /etc/mail/aliases files. You can also create and administer mailing lists that use the same alias files.

Depending on the configuration of your mail services, you can administer aliases by using the NIS or NIS+ name service to maintain a global aliases database. Otherwise, you could update all the local /etc/mail/aliases files to keep the aliases synchronized.

Users can also create and use aliases. Users can create aliases either in their local ~/.mailrc file, which only the user can use, or in their local /etc/mail/aliases file, which anyone can use. Users cannot normally create or administer NIS or NIS+ alias files.

Hardware Components

You can provide the three required elements of mail configuration in the same system
or have separate systems provide these elements.

When users are to communicate with networks outside your domain, you must also
add a fourth element, a mail gateway. For more information, refer to Mail Gateway.
The following sections describe each hardware component.

Mail Host

A mail host is the machine that you designate as the main mail machine
on your network. A mail host is the machine to which other systems
at the site forward mail that cannot be delivered. You designate a system
as a mail host in the hosts database by adding the word
mailhost to the right of the IP address in the local /etc/hosts file. Alternately,
you can add the word mailhost similarly to the hosts file in the
name service. For detailed task information, refer to How to Set Up a Mail Host in Chapter 13, Mail Services (Tasks).

Some sites use standalone machines that are not networked in a time-sharing configuration.
Specifically, the standalone machine serves terminals that are attached to its serial ports.
You can set up electronic mail for this configuration by designating the standalone system
as the mail host of a single-system network. Overview of the Hardware Components in Chapter 12, Mail Services (Overview) provides a
figure that shows a typical email configuration.

Mail Server

A mailbox is a single file that contains email for a particular user.
Mail is delivered to the system where the user's mailbox resides, which can
be on a local machine or a remote server. A mail server is any
system that maintains user mailboxes in its /var/mail directory. For task information, refer to
How to Set Up a Mail Server in Chapter 13, Mail Services (Tasks).

The mail server routes all mail from a client. When a client sends
mail, the mail server puts the mail in a queue for delivery. After
the mail is in the queue, a user can reboot or turn
off the client without losing those mail messages. When the recipient gets mail from
a client, the path in the From line of the message contains the
name of the mail server. If the recipient responds, the response goes to
the user's mailbox. Good candidates for mail servers are systems that provide a
home directory for users or systems that are backed up regularly.

If the mail server is not the user's local system, users in configurations
that use NFS software can mount the /var/mail directory by using the /etc/vfstab file,
if they have root access. Otherwise, users can use the automounter. If NFS
support is not available, users can log in to the server to read
their mail.

If users on your network send other types of mail, such as audio
files or files from desktop publishing systems, you need to allocate more space
on the mail server for mailboxes.

By establishing a mail server for all mailboxes, you can simplify your process of
doing backups. Backups can be difficult to do when mail is spread over
many systems. The disadvantage of storing many mailboxes on one server is that
the server can be a single point of failure for many users. However,
the advantages of providing good backups usually make the risk worthwhile.

Mail Gateway

The mail gateway is a machine that handles connections between networks that run different
communications protocols or communications between different networks that use the same protocol. For example,
a mail gateway might connect a TCP/IP network to a network that runs
the Systems Network Architecture (SNA) protocol suite.

The simplest mail gateway to set up is the mail gateway that connects
two networks that use the same protocol or mailer. This system handles mail
with an address for which sendmail cannot find a recipient in your domain.
If a mail gateway exists, sendmail uses the gateway to send and
receive mail outside your domain.

You can set up a mail gateway between two networks that use unmatched
mailers, as shown in the next figure. To support this configuration, you must
customize the sendmail.cf file on the mail gateway system, which can be
a difficult and time-consuming process.

Figure 14-1 Gateway Between Different Communications Protocols

If you have a machine that provides connections to the Internet, you can
configure that machine as the mail gateway. Carefully consider your site's security needs
before you configure a mail gateway. You might need to create a firewall
gateway between your corporate network and other networks, and set up that gateway
as the mail gateway. For task information, refer to How to Set Up a Mail Gateway in Chapter 13, Mail Services (Tasks).